Court decision to free Ukraine's opposition leader is unacceptable to prosecutor general who seeks pre-trial detention.

Ukraine's prosecutor general has said Kiev court's decision to free opposition leader Mikheil Saakashvili from pre-trial detention will be appealed.

Saakashvili, who was Georgia's president before he entered Ukrainian politics, must be placed under house arrest, said Yuri Lutsenko, Ukraine's prosecutor general, late on Monday.

"I can't imagine a country where a stateless person breaks through a border crossing, then organises a movement to overturn a government and nonetheless a judge releases him. We will appeal," he told ICTV, a Ukrainian broadcaster.

Saakashvili, the 49-year-old leader of Ukraine's Movement of New Forces opposition party, was arrested on December 9 in a Kiev apartment on charges, including cooperation with a criminal organisation.

When authorities tried to arrest him for the first time on December 5, his supporters freed him from a police van.

The development comes weeks after Saakashvili and his supporters launched mass anti-government protests, calling for an impeachment of President Petro Poroshenko, who they accuse of corruption.

Poroshenko brought Saakashvili and his team to Ukraine in 2015, gave him the Ukrainian citizenship and appointed him as the governor of the Odessa region to use his image as the author of Georgia's successful reforms.

But a year later, Saakashvili accused him of protecting corrupt oligarchs and resigned, moving to the opposition.

He was later stripped of his Ukrainian citizenship, becoming stateless as he had lost his Georgian citizenship to gain the Ukrainian one.

Call for new protests

On Monday, he said the court decision to free him would hasten Poroshenko's ouster, calling on his supporters to stage a mass protest on Sunday.

"Let's gather on Sunday and show that there are many of us and our numbers will grow. I am sure that the Ukrainian government has even less time left," he told his supporters. "The court decision was a slap in the face for the government."

Ukraine ousted pro-Russian President Viktor Yanukovich in February 2014 after months of bloody anti-government protests.

One of the main reasons for the revolution was the wish to rid the country of endemic corruption, but almost four years since the event, Ukrainians say they are yet to see improvement.

Saakashvili claims the current government is not interested in investigating past economic crimes or preventing future episodes as it benefits from them.